Enzyme-based photoelectrochemical cell for electric current generation

ABSTRACT

The invention provides a photobiological fuel cell converting chemical energy possessed by a carbon-containing compound and light energy into electrical energy. A positive electrode ( 24 ) and a negative electrode ( 23 ) provided with an electrolyte ( 26 ) interposed between them are arranged as constituents. An electromotive force is generated across the positive electrode ( 24 ) and the negative electrode ( 23 ) by an oxidation reaction involving an electrochemical reception of electrons from carbon-containing compound by an external electric circuit via the intermediacy of a photosensitizer molecule excited by light, an oxidation-reduction mediator, catalytic enzymes, and reduction reactions at a positive electrodes ( 24 ).

FIELD OF THE INVENTION

The present invention relates to a photobiological fuel cell utilizing aphotoelectrochemical reaction and an enzymatic action which incombination perform generation of electricity by the action of aphotosensitizer compound which produces an oxidant species and anelectron when irradiated with light, an oxidation-reduction mediatorwhich supplies an electron to the oxidant species, and one or moreenzymes that oxidize a carbon-containing compound and reduce theoxidation-reduction mediator to return it to its original form. Thecarbon-containing compound may be one that is cyclically regenerated ina natural biosphere, such as carbohydrate (e.g., sugar, starch), lipid,hydrocarbon, alcohol, aldehyde or organic acid.

BACKGROUND OF THE INVENTION

A carbon-containing compound which is cyclically regenerated in anatural biosphere such as carbohydrate (e.g., sugar, starch), lipid,hydrocarbon, alcohol, aldehyde or organic acid is produced from carbondioxide gas and water by photosynthesis. Solar energy, which has beenconverted to chemical energy and accumulated in the form ofcarbon-containing compounds, is used as chemical energy in themetabolism of an organism to produce carbon dioxide gas and water. Thisforms a clean cyclic system. For example, a carbohydrate, which is ageneral term for saccharides such as monosaccharide, oligosaccharide andpolysaccharide as well as saccharide analogues such as cyclic polyvalentalcohols and amino saccharides, is made by a plant or otherphotosynthetic organism during photosynthesis. A second organism caningest a carbohydrate from a plant and use it as a food to provideenergy. When glucose, which is a typical carbohydrate represented by thechemical formula C₆H₁₂O₆, is completely oxidized, it releases 24electrons per molecule, and is converted to carbon dioxide gas. In thebody of an animal or other organism, the potential energy of the 24electrons is utilized as an energy source. A thermodynamic calculationshows that glucose has a potentially useable energy of 2,872 kJ per mol,or 4.43 Wh per g. This energy density exceeds the weight energy density(3.8 Wh/g) of metallic lithium, which is used as a negative electrodefor a lithium battery, which is known as a high energy density battery.

There are several methods for utilizing the chemical energy possessed bya carbon-containing compound such as a carbohydrate. One such methodcomprises direct combustion of a carbon-containing compound in air togive heat energy. Another method uses a carbon-containing compound as anutrient in fermentation, wherein a microorganism produces fuels such asmethane or ethanol. Yet another method produces an energy-rich compoundsuch as ATP through the mediation of enzymes present in an organism.

As one method for utilizing the chemical energy possessed by acarbon-containing compound, there is a biological fuel cell disclosed inU.S. Pat. No. 6,294,281 that uses an enzyme and an oxidation-reductionmediator. Further, U.S. Pat. No. 4,117,202 discloses aphotosynthetically driven biological fuel cell, which generateselectricity using a photosynthetic cell derived from a living organism(Digitaria sanguinalis) that uses a carbon-containing compound as anutrient.

The photosynthetically driven biological fuel cell disclosed in U.S.Pat. No. 4,117,202 can utilize chemical energy possessed by acarbon-containing compound as well as light radiation energy such assunlight. This may be an improvement compared with the process disclosedin U.S. Pat. No. 6,294,281, which can utilize only chemical energypossessed by a carbon-containing compound. However, thephotosynthetically-driven biological fuel cell disclosed in U.S. Pat.No. 4,117,202 utilizes a photosynthetic cell originated from a livingorganism, and thus requires careful control over temperature, solutionformulation, and nutrient to allow the living cells to survive. Thisfuel cell is also disadvantageous in that a culture vessel for livingcells must be used, requiring a complicated and large power-generatingapparatus. The biological fuel cell disclosed in U.S. Pat. No. 6,294,281can generate electric power merely by dipping positive and negativeelectrodes having an enzyme and an oxidation-reduction mediator fixedthereto into an electrolyte containing a carbon-containing compound.Thus, this biological fuel cell is advantageous in that it requires onlya simplified power-generating system and can be miniaturized. However,this biological fuel cell cannot utilize light radiation energy.

SUMMARY OF THE INVENTION

It is therefore an aim of the invention to provide a photobiologicalfuel cell which can be miniaturized and utilize light energy in additionto chemical energy possessed by a carbon-containing compound.

The foregoing aim of the present invention will become apparent from thefollowing detailed description and examples.

The invention provides a photobiological fuel cell comprising a positiveelectrode and a negative electrode provided with an electrolyteinterposed between them. Through the action of several intermediaryspecies, the negative electrode receives an electron from acarbon-containing fuel compound, which is therefore oxidized. Thecarbon-containing fuel compound may be one that is cyclicallyregenerated in a natural biosphere, such as carbohydrate (e.g., sugar,starch), lipid, hydrocarbon, alcohol, aldehyde or organic acid. Thisoxidation is accomplished through several mediators. The first of theseis a photosensitizer material, attached to the negative electrode, thatis converted to an electronically excited state when irradiated withlight. The electronically excited state injects an electron into theelectrode, from where it flows into an external circuit to do work. Lossof an electron from the photosensitizer material leaves an oxidizedphotosensitizer, which is in turn reduced back to its original form byan oxidation-reduction mediator. The resulting oxidized form of theoxidation-reduction mediator is reduced back to its original form by oneor more enzymes, which obtain the necessary electrons by oxidation ofthe carbon-containing fuel compound. Thereby, chemical energy possessedby the carbon-containing fuel compound and light energy such as sunlightcan be utilized to generate electricity.

The positive electrode may be one that undergoes an oxygen reductionreaction at a higher potential (i.e., more anodic potential) than theoxidation reaction at the negative electrode, or another suitableelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example and to make the description more clear, reference ismade to the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating the procedure ofphotoelectrochemical oxidation of a carbon-containing compound (fuel) bya photosensitizer (S) in the presence of an enzyme and anoxidation-reduction mediator (R) of the invention;

FIG. 2 is a diagram illustrating the structure of a power-generatingcell used in the evaluation of the photoelectrochemical properties andbattery properties in an example of the invention, wherein the referencenumeral 21 indicates a power-generating cell, the reference numeral 22indicates a silicon plug, the reference numeral 23 indicates a negativeelectrode, the reference numeral 24 indicates a counter electrode, thereference numeral 25 indicates a reference electrode, the referencenumeral 26 indicates an electrolyte, and the reference numeral 27indicates an air electrode;

FIG. 3 is a diagram illustrating the current-voltage characteristics ofthe power-generating cell in another example of the invention;

FIG. 4 is a diagram illustrating the change of NADH concentration withtime a solution containing various combinations of enzymes and methanol;

FIG. 5 is a diagram illustrating the relationship between the amount ofNADH consumed in the electrolyte in the power-generating cell and theamount of electrons removed from the power-generating cell by theexternal circuit in a further example of the invention;

FIG. 6 is a diagram illustrating the relationship between the consumedamount of NADPH in the electrolyte in the power-generating cell and theamount of electrons removed from the power-generating cell by theexternal circuit in a further example of the invention;

FIG. 7A is a diagram illustrating the change of NADH concentration withtime in the electrolyte in the power-generating cell in a furtherexample of the invention;

FIG. 7B is a diagram illustrating the change of NADH concentration withtime in the electrolyte in the power-generating cell in a furtherexample of the invention; and

FIG. 8 is a diagram illustrating the relationship between the amount ofNADH consumed in the electrolyte in the power-generating cell and theamount of electrons taken out of the power-generating cell by theexternal circuit in a further example of the invention.

DETAILED DESCRIPTION OF DRAWINGS

The photobiological fuel cell of the invention includes a positiveelectrode and a negative electrode provided as constituents with anelectrolyte interposed between them. When the negative electrodereceives electrons from a carbon-containing fuel compound via aphotosensitizer compound which produces an oxidant and electrons whenirradiated with light, an oxidation-reduction mediator which supplieselectrons to the oxidized photosensitizer molecule, and enzymes thatcatalyze oxidation of the carbon-containing fuel compound, anelectromotive force is generated across the positive electrode and thenegative electrode. This makes it possible to utilize chemical energyaccumulated in the carbon-containing compound directly as electricalenergy in a form biased by light radiation energy.

FIG. 1 illustrates a schematic diagram of the configuration of thephotobiological fuel cell of the invention. FIG. 1 illustrates howelectrons (e⁻) which have been possessed by a carbon-containing compound(fuel) are released from the carbon-containing compound and ultimatelyflow through an external electric circuit (load) from the negativeelectrode to the positive electrode. The electron flow within the celloccurs via a photosensitizer compound (S) retained in or on an oxidesemiconductor (MeOx) shown in a spherical form. When irradiated withlight, sensitizer S produces an excited state S*. The excited state S*injects an electron into the semiconductor, leaving an oxidant S⁺. Theelectron (e⁻) has been biased by light irradiation to the opticalexcitation energy of S (the difference in energy between S and S*). Theelectron does work in the external circuit, and reaches the positiveelectrode, where it is used in a reduction reaction with material M. Inthis manner, an electromotive force is generated across the positiveelectrode and the negative electrode, generating electricity. During theprocess, the oxidized photosensitizer S⁺ is reduced to its original formby a redox mediator R, generating R in an oxidized form. The enzyme(s)reduce the oxidized redox mediator back to its original form, obtainingthe necessary electrons from oxidation of the carbon-containing fuelcompounds. Thus, neither photosensitizer S nor redox mediator R areconsumed.

The photosensitizer compound S, which produces an oxidant S+ and anelectron when irradiated with light, is disposed on an oxidesemiconductor. This compound may have a single light absorption peak ora plurality of light absorption peaks in a wavelength range of ˜300 nmto ˜1,000 nm. Such a compound may be a metal complex dye, organic dye,or the like. Examples of a metal complex dye are ruthenium complex dyesor platinum complex dyes having biquinoline, bipyridyl, phenanthroline,or thiocyanic acid or derivatives thereof as ligands. Examples oforganic dyes which may also contain metal atoms are porphyrin-based dyeshaving a single porphyrin ring or a plurality of porphyrin rings. Theporphyrin rings may be metal free, or contain zinc (Zn), magnesium (Mg),or the like as a central atom. Examples of such a porphyrin-based dyeinclude those represented by the general formulae P1 to P6 below.Examples of organic dyes are 9-phenylxanthene-based dyes,merocyanine-based dyes, polymethine-based dyes, or the like. Inparticular, the compound P1,5-(4-carboxyphenyl)-10,15,20-(4-methylphenyl) porphyrin, having thestructure shown below has a high light absorption efficiency and alsohigh affinity for the oxide semiconductor so that it cannot be easilyeluted with the electrolyte and thus can be kept stable even after aprolonged contact with the electrolyte. Further, since generation ofexcited electrons from the compound P1 by irradiation with light canoccur over a prolonged lifetime, the compound P1 can exhibit highphotoelectric conversion efficiency to advantage. The disposition of acompound which produces an oxidant and an electron when irradiated withlight on an oxide semiconductor makes it possible to quickly move anexcited electron produced by irradiation with light to the oxidesemiconductor and makes less likely the recombination of the oxidant andexcited electron produced by irradiation with light, thereby keeping theefficiency of reception of electrons from the carbon-containing compoundinto the external electrical circuit higher.

As the oxide semiconductor, tin dioxide (SnO₂), titanium dioxide (TiO₂),zinc oxide (ZnO), tungsten oxide (WO₃) or composites thereof such asTiO₂—WO₃ may be used.

Examples of the carbon-containing fuel compound used in this invention,which may be one that is cyclically regenerated in a natural biosphere,include carbohydrates (e.g., sugars, starches), lipids, hydrocarbons,alcohols, aldehydes and organic acids. Such compounds can be producedfrom carbon dioxide gas and water by photosynthesis and thenaccumulated. The solar energy stored in these compounds is used aschemical energy via the metabolism of a organism, producing carbondioxide gas. This forms a clean cyclic system.

Examples of the oxidation-reduction mediator (R) which receiveselectrons from the carbon-containing fuel compound through the mediationof enzymes and supplies electrons to the oxidant (S⁺) produced byirradiation with light to regenerate the original photosensitizercompound (S) are a quinone/hydroquinone oxidation-reduction couple, theNAD⁺/NADH oxidation-reduction couple, the NADP⁺/NADPHoxidation-reduction couple, the I₂/I₃ ⁻ oxidation-reduction couple, andmetal proteins having an oxidation-reduction capacity such as ferredoxinand myoglobin.

The enzymes, which catalyze the transfer of electrons from thecarbon-containing fuel compound to an oxidized form of theoxidation-reduction mediator R, are not specifically limited. Inpractice, however, dehydrogenase enzymes may be used singly or incombination depending on the kind of the carbon-containing fuelcompound. In the case where the fuel is glucose, an enzyme systemcontaining at least glucose dehydrogenase (GDH) may be used.

In the case where the fuel is D-glucose-6-phosphate, an enzyme systemcontaining at least D-glucose-6-phosphate dehydrogenase (G-6-PDH) or atleast G-6-PDH and 6-phosphogluconate dehydrogenase (6-PGDH) may be used.

In the case where the fuel is methyl alcohol, an enzyme systemcontaining at least an alcohol dehydrogenase (ADH), an enzyme systemcontaining at least ADH and an aldehyde dehydrogenase (ALDH), or anenzyme system containing at least ADH, ALDH and a formate dehydrogenase(FDH) may be used.

In the case where the fuel is ethyl alcohol, an enzyme system containingat least an alcohol dehydrogenase (ADH) or an enzyme system containingat least ADH and an aldehyde dehydrogenase (ALDH) may be used. In thecase where a plurality of fuels is used, enzymes corresponding to thesefuels may be used in admixture.

As the electrolyte to be incorporated into the photobiological fuel cellof the invention there may be used any material regardless of whether itis an organic material, inorganic material, liquid or solid so far as itallows the movement of anions and/or cations from the positive electrodeto the negative electrode and/or from the negative electrode to thepositive electrode to cause continuous progress of oxidation-reductionreactions at the positive electrode and the negative electrode. Anaqueous solution obtained by dissolving a salt such as KCl, NaCl, MgCl₂,NH₄Cl and Na₂HPO₄, an alkali such as NH₄OH, KOH and NaOH or an acid suchas H₃PO₄ and H₂SO₄ in water is safe, causes no environmental pollution,and can be easily handled to advantage. Alternatively, a solution of aquaternary ammonium salt such as pyridinium iodide, a lithium salt suchas lithium iodide, an imidazolium salt such as imidazolinium iodide,t-butylpyridine or the like in acetonitrile, methoxyacetonitrile ormethoxypropionitrile, an ion exchange membrane made of a polymermaterial such as fluororesin having sulfonic acid groups, amide groups,ammonium groups, pyridinium groups or the like or a polymer electrolytesuch as solution of a salt such as LiBF₄, LiClO₄ and (C₄H₉)₄NBF₄ in apolypropylene oxide, polyethylene oxide, acrylonitrile, polyvinylidenefluoride, polyvinyl alcohol or the like may be used.

The reaction at the positive electrode in the photobiological fuel cellof the invention involves a reduction reaction occurring at a higher (ormore anodic) potential than that of the electron taken out of thecarbon-containing compound via an optically excited active species (S*)of molecule at the negative electrode. Any reduction reaction can beemployed so far as the electron thus taken out is electrochemicallyreceived by the positive electrode via the external load.

Examples of the reaction at the positive electrode include reductionreactions of water or oxygen, reduction reactions of hydroxide or oxidessuch as NiOOH, MnOOH, Pb(OH)₂, PbO, MnO₂, Ag₂O, LiCoO₂, LiMn₂O₄ andLiNiO₂, reduction reactions of sulfides such as TiS₂, MoS₂, FeS andAg₂S, reduction reactions of metal halides such as AgI, PbI₂ and CuCl₂,reduction reactions of halogen such as Br₂ and I₂, reduction reactionsof organic sulfur compounds such as quinone and organic disulfidecompounds, and reduction reactions of electrically-conductive polymerssuch as polyaniline and polythiophene.

In particular, the positive electrode is preferably an oxygen electrodefor reducing oxygen. In this arrangement, a gas containing oxygen can beused as the positive active material, eliminating the necessity ofretaining a positive active material in the battery and hence making itpossible to form a battery having a higher energy density.

Any material capable of reducing oxygen may be used as the oxygenelectrode. Examples of such an oxygen-reducing material includeactivated charcoal, manganese oxide including MnO₂, Mn₃O₄, Mn₂O₃ andMn₅O₈, platinum, palladium, iridium oxide, platinum-ammine complexes,cobalt-phenylenediamine complexes, metal porphyrins (metal: cobalt,manganese, zinc, magnesium, etc.), and perovskite oxides such asLa(Ca)CoO₃ and La(Sr)MnO₃.

The invention will be further described in the following examples.

EXAMPLE 1

As a photosensitizer compound which produces an oxidant and an electronwhen irradiated with light,5-(4-carboxyphenyl)-10,15,20-(4-methylphenyl)porphyrin (P1) was used asa typical representative of a porphyrin photosensitizer used to preparea negative electrode.

Preparation of the Negative Electrode

A light-transmitting glass substrate with a thickness of 1 mm bearing athin film of electrically conducting indium-tin oxide (ITO) with asurface resistivity of 10-12 Ω/cm² was used to prepare the negativeelectrode. A 1% by weight aqueous dispersion of particulate tin dioxide(SnO₂) having an average particle diameter of 10 nm was deposited on theITO film by spraying or otherwise applying layers over a hot plate. Theelectrode was dried at a temperature of 80° C., and then sintered at atemperature of 400° C. in air for 1 hour to form a film of particulateSnO₂. Subsequently, the electrode was dipped into a 1-5 mM solution ofphotosensitizer P1 (dissolved in dichloromethane, toluene, or hexanes)for typically 1 hour, withdrawn from the solution, washed with cleansolvent, and dried with a stream of nitrogen gas. The presence of P1 onthe electrode particulate surface was confirmed by its absorptionspectrum. In this manner, the negative electrode was prepared.

Assembly of Test Cell

The negative electrode thus prepared was then used to assemble apower-generating cell 21 having the structure shown in FIG. 2.

In the power-generating cell 21, the film of particulate SnO₂ on thenegative electrode 23 on which the dye P1 is deposited comes in contactwith an electrolyte 26. In the electrolyte 26 are disposed a counterelectrode 24 which forms a battery in combination with the negativeelectrode 23 and a reference electrode 25 which gives a referencepotential on the basis of which the potential of the negative electrode23 is measured. Further disposed is an air electrode 27, which forms abattery in combination with the negative electrode 23 instead of thecounter electrode 24. The air electrode 27 was prepared by embedding amixture of Mn₂O₃ powder, activated charcoal powder, acetylene blackpowder and polytetrafluoroethylene (PTFE) binder on a nickel screenhaving a thickness of 0.2 mm. The reference numeral 22 indicates asilicon plug for fixing the counter electrode 24 and the referenceelectrode 25 to the power-generating cell 21.

Operating Characteristics of Photoelectric Power-Generating Cell

A power-generating cell (a) was assembled as described above, using thenegative electrode 23, a platinum (Pt) counter electrode (24), and anelectrolyte 26, which is a 0.1 M aqueous solution of sodium acetate(NaOAc) containing 2.5 mM hydroquinone (QH₂) as an oxidation-reductionmediator (R).

A power-generating cell (b) was assembled as described above, using thenegative electrode 23, a platinum (Pt) counter electrode 24 immersed ina saturated aqueous solution of potassium sulfate free of dissolvedoxygen and isolated from the electrolyte 26 by an ion-permeablemembrane, and electrolyte 26, which is a 0.1 M aqueous solution ofsodium acetate (NaOAc) containing 2.5 mM nicotinamide-adeninedinucleotide in the reduced form (NADH) as an oxidation-reductionmediator (R).

A power-generating cell (c) was assembled as described above, using thenegative electrode 23, a counter electrode 24, which is amercury/mercury (I) sulfate electrode separated from electrolyte 26 byan ion permeable membrane, and electrolyte 26, which is an 0.1 M aqueoussolution of sodium acetate containing 2.5 mM nicotinamide-adeninedinucleotide in the reduced form (NADH) as an oxidation-reductionmediator (R).

FIG. 3 illustrates current-voltage characteristics of thesepower-generating cells developed when they are irradiated with lighthaving a wavelength of 520 nm. In FIG. 3, the curves (a), (b) and (c)indicate the current-voltage characteristics of the power-generatingcells (a), (b) and (c), respectively. All the power-generating cells(a), (b) and (c) work as batteries, although they show differences incurrent-voltage characteristics. In FIG. 3, the curve (d) indicates thecurrent-voltage characteristics of the power-generating cell (a)developed when it is not irradiated with light. When not irradiated withlight, a power-generating cell gives little or no output current.

In these cells, the dye deposited on the negative electrode acts as aphotosensitizer compound (S) which upon irradiation produces an excitedstate (S*). In contact with the metal oxide, it injects an electron intothe oxide particle, producing an oxidant (S⁺). The external circuitremoves the electron thus produced, where it is then measured as outputcurrent of the battery. In cells of type (c), the oxidant (S⁺) receivesan electron from the oxidation-reduction mediator NADH (or in some casesQH₂), regenerating S. Thus, in a cell lacking enzymes orcarbon-containing fuel compounds, the supply of electrons to theexternal circuit lasts until NADH (or QH₂ when that is used as theoxidation-reduction mediator) is consumed.

An assay (not performed in the cell) was done to test the production ofNADH when NAD⁺ is present and methanol is used as the carbon-containingfuel compound. An aqueous solution of pH 8.0 containing 1 M NaCl, 5 mMoxidized nicotinamide-adenine-dinucleotide (NAD⁺) and methanol with 0.0and 0.05 mM reduced nicotinamide-adenine-dinucleotide (NADH) and analcohol dehydrogenase (ADH), an aldehyde dehydrogenase (ALDH) and aformate dehydrogenase (FDH) as enzymes added thereto. The change of NADHconcentration with time during irradiation with light is shown in FIG.4.

In FIG. 4, the symbol ▴ indicates the change of NADH concentration in asolution having 5.0 mM NAD⁺, 0.05 mM NADH, ADH, ALDH and FDH addedthereto. The symbol ● indicates the change of NADH concentration in asolution having 5 mM NAD⁺, 0.05 mM NADH and ADH added thereto. Thesymbol ◯ indicates the change of NADH concentration in the same solutionas in (●), but having ALDH added thereto after the lapse of apredetermined time from the addition of NADH and ADH. The symbol ◯+indicates the change of NADH concentration in the solution (◯) buthaving FHD added thereto after the lapse of a predetermined time fromthe addition of NADH, ADH and ALDH. The symbol ▪ indicates the change ofNADH concentration in the electrolyte having 5 mM NAD⁺ and ADH addedthereto. The symbol □ indicates the change of NADH in the same solutionas the solution ▪ but having ALDH added thereto. The symbol □+ indicatesthe change of NADH in the same solution as the electrolyte □ but havingFDH added thereto. All these tests were observed to have an increase ofNADH concentration, demonstrating that an electron moves from methanolto NAD⁺ through the mediation of the enzyme to produce NADH. In otherwords, as shown in FIG. 4, NADH is formed from NAD⁺ through themediation of enzymes that utilize methanol. These results imply that aslong as methanol is present in a power-generation cell with themediation of enzymes, NADH used by the negative electrode will beregenerated, and power generation can be maintained under irradiationwith light.

FIG. 5 is a graph illustrating the relationship between the amount ofelectrons taken out of the cell by the external circuit (abscissa) andthe amount of NADH consumed by irradiation with light (ordinate) in apower-generating cell of type (c), with the electrolyte 26 containingNADH and methanol. The symbol ◯ indicates the relationship between theamount of NADH consumed and the amount of electrons produced in theexternal circuit when the electrolyte is free of enzymes. Thisrelationship shows that the consumed amount of NADH and the amount ofelectrons are proportional to each other, demonstrating that electronsreleased from NADH are properly taken out by the external circuit. Thesymbol ● indicates the relationship between the amount of NADH consumedand the amount of electrons produced in the external circuit when ADH,ALDH and FDH are added to the electrolyte as the enzyme system. Underthese conditions, little or no NADH is consumed, regardless of theamount of electrons removed by the external circuit. In other words,NADH releases electrons to form NAD⁺, which then receives electrons frommethanol through the mediation of the enzymes thus added to regenerateNADH. This state of little or no NADH consumption lasts as long asmethanol is present in the electrolyte. In other words, power generationcontinues while methanol is present.

In these and other experiments, the concentration of NADH in theelectrolyte was determined by the intensity of the peak present in thevicinity of 340 nm in the UV absorption spectrum of NADH.

In the present example, tin oxide (SnO₂) was used as the oxidesemiconductor. The same evaluation was made with particulate TiO₂, andcould be made with films of particulate metal oxide such as ZnO andTiO₂.WO₃ instead of SnO₂. The same evaluation using SnO₂ was also madeon the compounds P2, P3, P4, P5 and P6 instead of the compound P1 as aphotosensitizer. These compounds also produce an oxidant and an electronwhen irradiated with light. As a result, these power-generating cellswill exhibit operating characteristics similar to that of P1.

EXAMPLE 2

A power-generating cell was formed by the same type of negativeelectrode 23 as used in Example 1, platinum (Pt) as counter electrode 24and an aqueous buffered solution at pH 8.0 containing NADP⁺/NADPH as theoxidation-reduction mediator in the electrolyte. D-Glucose-6-phosphate(G-6-P) was used as a carbon-containing fuel compound.D-glucose-6-phosphate dehydrogenase (G-6-PDH) and 6-phosphogluconatedehydrogenase (6-PGDH) were used as an enzyme system.

FIG. 6 is a graph illustrating the relationship between the amount ofelectrons removed by the external circuit from the electrolyte(abscissa) and the amount of NADPH consumed during irradiation withlight (ordinate). The symbol ◯ indicates the relationship between theamount of NADPH consumed and the amount of electrons injected into theexternal circuit with an the electrolyte free of fuel and enzyme. Thisrelationship shows that the amount of NADPH consumed and the amount ofelectrons produced are proportional to one another, demonstrating thatelectrons released from NADPH are removed by the external circuit. Thesymbol ● indicates the relationship between the amount of NADPH consumedand the amount of electrons removed in the electrolyte to which has beenadded G-6-P as a carbon-containing fuel compound, and the enzymeG-6-PDH. The amount of NADPH consumed is greatly reduced. This resultshows that NADPH has released electrons to form NADP⁺, which thenreceives electrons from G-6-P to regenerate NADPH through the mediationof the added enzymes. The amount of NADPH consumed remains approximatelyconstant regardless of the number of electrons removed by the externalcircuit. This state lasts as long as G-6-P is present in theelectrolyte. At later times, when G-6-P is entirely oxidized togluconolactone-6-phosphate, the amount of NADPH consumed again rises.The gluconolactone-6-phosphate hydrolyzes in the electrolyte to6-phosphogluconate (6-PG). The amount of NADPH consumed increases inproportion to the amount of electrons removed into the external circuit.When 6-PGDH, which is an enzyme that oxidizes 6-PG, is added to theelectrolyte, the amount of NADPH consumed shows a sudden drop as shownby the symbol ◯+ in FIG. 6. Electrons are again supplied by 6-PG, whichis a fuel, to regenerate NADPH. The reception of electrons by theexternal circuit lasts as long as 6-PG is present in the electrolyte.

In this example, the oxide semiconductor was tin oxide (SnO₂). Similarevaluations could be made on films of particulate TiO₂ or otherparticulate metal oxides such as ZnO and TiO₂.WO₃.

EXAMPLE 3

A power-generating cell was formed by the same negative electrode 23 asused in Example 1, platinum (Pt) as a counter electrode 24 and abuffered solution at pH 8.0 containing a 0.5 mM NADH and 10 mM NAD⁺ asan electrolyte. Ethanol (CH₃CH₂OH) was used as a carbon-containing fuelcompound. The nicotinamide-adenine-dinucleotide couple (NADH)/(NAD⁺) wasused as the oxidation-reduction mediator. An alcohol dehydrogenase (ADH)and an aldehyde dehydrogenase (ALDH) were used as an enzyme system.

FIG. 7A is a graph illustrating the change of NADH concentration withtime in the electrolyte containing ethanol. An alcohol dehydrogenase(ADH) enzyme system was added thereto after 60 minutes. In FIG. 7A, thesymbol ▾ indicates the change of NADH concentration in thepower-generating cell containing ethanol and ADH. The concentration ofNADH increases with light irradiation time until it reaches a valuedetermined by the amount of electrons removed by the external circuitand the amount of ethanol in the electrolyte. On the contrary, thesymbol ● indicates the change of NADH concentration in an identicalelectrolyte put in a container free of electrodes, but having ADH addedthereto under the same conditions as in the power-generating cell. InFIG. 7A, these data are noted as “Control”. Since the Control has noelectrodes, electrons are not removed by the external circuit. The NAD⁺in the electrolyte receives electrons from ethanol and is converted toNADH through the mediation of ADH. Thus, the concentration of NADHcontinues to increase with time.

A power-generating cell was formed by a negative electrode 23, an airelectrode 27 and an electrolyte at pH 8 containing ethanol, NADH and ADHas an enzyme system. When irradiated with sunlight, the power-generatingcell operated as a photobiological fuel cell having a voltage of about0.65 V.

FIG. 7B is a graph illustrating the change of NADH concentration withtime in the electrolyte containing ethanol with later addition of analcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) as anenzyme system.

In FIG. 7B, the gray empty squares indicate the NADH concentration inthe power-generating cell after addition of ethanol, but prior toaddition of ADH and ALDH. The NADH concentration is constant. Afteraddition of ADH and ALDH (gray filled squares), the concentration ofNADH increases until it approaches a value determined by the amount ofelectrons taken out by the external circuit and the amount of ethanoland its oxidation product acetaldehyde in the electrolyte. The blackempty squares indicate the change of NADH concentration in anelectrolyte with ethanol, put in a container free of electrodes, priorto addition of enzymes. The concentration is constant. After addition ofADH and ALDH under the same conditions as in the power-generating cell,the rise in NADH concentration is shown as black filled squares. In FIG.7B, these latter plots are identified as “Control”. Since Control has noelectrodes, electrons are not removed by the external circuit. NAD⁺ inthe electrolyte receives electrons from ethanol and acetaldehyde, and isconverted to NADH through the mediation of ADH and ALDH. Thus, theconcentration of NADH continues to increase with time.

A power-generating cell was formed by an electrolyte 23, an airelectrode 27, and an electrolyte containing NADH, NAD⁺, ethanol, and ADHand ALDH as an enzyme system. When irradiated with sunlight, thepower-generating cell operated as a photobiological fuel cell having avoltage of about 0.65 V.

In the present example, tin oxide (SnO₂) was used as the oxidesemiconductor. The same evaluation could be made with films ofparticulate TiO₂, or other particulate metal oxides such as ZnO andTiO₂.WO₃ instead of SnO₂. The same evaluation could also be made on thecompounds P2, P3, P4, P5 and P6 instead of the compound P1 as aphotosensitizer compound, which produces an oxidant and an electron whenirradiated with light.

EXAMPLE 4

A power-generating cell was formed by the same negative electrode 23 asused in Example 1, platinum (Pt) as a counter electrode 24 and a buffersolution having pH 7.3 containing an NAD⁺/NADH oxidation-reductionmediator in the electrolyte. D-glucose was used as a carbon-containingcompound was used. D-glucose-dehydrogenase (GDH) was used as an enzyme.

FIG. 8 is a graph illustrating the relationship between the amount ofelectrons removed by the external circuit from the electrolyte havingGDH added thereto as an enzyme system (abscissa) and the amount of NADHconsumed during irradiation with light (ordinate). The symbol ◯indicates the relationship between the amount of NADH consumed and theamount of electrons produced prior to addition of the enzyme, but withD-glucose present. This relationship shows that the amount of NADHconsumed and the amount of electrons are proportional to one another,demonstrating that electrons released from NADH are removed by theexternal circuit. The symbol ● indicates the relationship between theamount of NADH consumed and the amount of electrons produced in theexternal circuit from the electrolyte after addition of the enzymesystem.

Before addition of the enzyme, NADH is consumed and oxidized to NAD⁺,with the concurrent production of electrons in the external circuit.After addition of the enzyme, NADH is regenerated and the amountapparently consumed drops slightly below the original amount. In otherwords, NADH releases electrons to form NAD⁺, which then receiveselectrons from D-glucose through catalysis by the enzyme to reform NADH.Thus, in the presence of the enzyme, the amount of NADH consumed is keptconstant regardless of the number of electrons thus taken out. Thisstate lasts as long as D-glucose is present in the electrolyte.

In the present example, tin oxide (SnO₂) was used as the oxidesemiconductor. The same evaluation could be made on films of particulateTiO₂, and on other particulate metal oxides such as ZnO and TiO₂.WO₃,instead of SnO₂. The same evaluation could also be made on the compoundsP2, P3, P4, P5 and P6 instead of the compound P1 as a photosensitizercompound which produces an oxidant and an electron when irradiated withlight.

As mentioned above, the invention provides a photobiological fuel cell,which carries out an oxidation reaction involving the electrochemicalreception of electrons from a carbon-containing fuel compound by anexternal electric circuit, via a photosensitizer molecule opticallyexcited at the negative electrode and suitable electron mediators, togenerate an electromotive force across the positive electrode and thenegative electrode. In accordance with the invention, chemical energypossessed by the carbon-containing compound can be effectively utilizedas electrical energy.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made thereto withoutdeparting from the spirit and scope thereof.

1. A photobiological fuel cell comprising a positive electrode and anegative electrode provided with an electrolyte interposed between them,wherein the reactions of and concerning the negative electrode comprise:(1) a process that comprises irradiation with light to produce anexcited state of a photosensitizer, which provides an electron and anoxidized form of the photosensitizer; (2) a process that comprisessupplying an electron to the oxidized photosensitizer from a reducedform of an oxidation-reduction mediator; and (3) a process whichcomprises one or more oxidation reactions of carbon-containing compoundscatalyzed by one or more enzymes to provide electrons to an oxidizedform of said oxidation-reduction mediator, whereby the reaction of thecarbon containing compound as a fuel causes the supply of electric powerto an external electric circuit. 2-12. (canceled)